Modern gas turbine engine are often equipped with relatively complex rotor assemblies including multiple coaxial, gear-linked shafts supportive of a number of compressors, air turbines, and, in the case of turbofan engines, a relatively large intake fan. During high speed rotation of the rotor assembly, vibrations originating from rotor imbalances, bearing imperfections, de-stabilizing forces, and the like may be transmitted through the rotor bearings, to the engine case, and ultimately to the aircraft fuselage. When physically and/or acoustically perceptible, rotor-emitted vibrations transmitted to the aircraft fuselage can decrease passenger comfort. Rotor-emitted vibrations may also reduce the operational lifespan of the engine components, such as the rotor bearings, and degrade various measures of engine performance, such as thrust output and fuel efficiency. Rotor-emitted vibrations reach their highest amplitudes during rotor critical modes; that is, when the rotational frequency of the rotor assembly induces significant off-axis motion of the rotor assembly due to, for example, deflection or bending of the rotor assembly spool (referred to as “critical flex modes”) or rotor bearings eccentricies (referred to as “rigid body critical modes”). It is not uncommon for the rotor assembly of a multi-spool gas turbine engine to exhibit five or more critical modes distributed across the operational range of the gas turbine engine.
Active hydraulic devices referred to as squeeze-film dampers (“SFDs”) can be disposed around one or more of the rotor bearings to help reduce the magnitude of rotor-emitted vibrations transmitted to the engine casing and aircraft fuselage. SFDs are, however, limited in several respects. SFDs are characterized by non-linear damping profiles and are consequently capable of providing optimal vibration attenuation only over a relatively narrow frequency range. Thus, while an SFD can be tuned to provide peak damping at a single, targeted rotor critical mode, the SFD will typically provide less-than-optimal damping at other operational frequencies and through other rotor critical modes. Furthermore, as the rotor critical modes vary in conjunction with changing rotor imbalances, SFDs may become gradually less effective at attenuating vibrations over the operational lifespan of the gas turbine engine. As a further limitation, the stiffness and damping profiles of an SFD are inherently linked and cannot be independently tuned. As a result, it can be difficult to optimize the damping characteristics of an SFD without reducing the stiffness thereof and, in so doing, sacrificing some degree of rotor centerline control. Poor centerline control decreases the ability of the SFD to counteract static loading conditions (e.g., gravity sag) and generally requires the provision of larger tip clearances within the gas turbine engine, which reduces overall engine efficiency.
It is thus desirable to provide embodiments of a gas turbine engine including a broadband damping system having an increased damping bandwidth, as taken over the operational frequency range of the engine's rotor assembly, to more effectively attenuate vibrations emitted from the rotor assembly through multiple critical modes. Ideally, embodiments of such a broadband damping system would have a substantially linear damping profile to permit high damping through a broad range of frequencies and loading conditions, while also having a substantially linear and independently-tunable stiffness profile to improve rotor centerline control. It would also be desirable to provide of such a broadband damping system that could be utilized in conjunction with other types of turbomachinery, such as turbochargers. Finally, it would be desirable to provide embodiments of a method for producing a broadband-damped gas turbine engine. Other desirable features and characteristics of embodiments of the present invention will become apparent from the subsequent Detailed Description and the appended Claims, taken in conjunction with the accompanying drawings and the foregoing Background.